1. Field of the Invention
The present invention relates to a technique of correcting for a variation in the brightness distribution of a projection-type video display apparatus (also called a video projector in the following description). In particular, the invention is effective in correcting for such variations in mass-production of the projection-type video display apparatus.
2. Description of the Related Art
FIG. 8 is a block diagram showing a brightness correction circuit of a conventional projection-type display apparatus disclosed in Japanese Unexamined Patent Publication No. Sho. 63-268380. FIGS. 9-11 illustrate the operation of the circuit of FIG. 8.
In a video projection lens system of a video projector or the like, a lens barrel has a given length and the solid angle becomes smaller as the direction inclines more from the optical axis. Therefore, as shown in FIG. 11, what is called a shading phenomenon occurs in which the brightness in a peripheral portion CR of a screen 100 is lower than in a central portion, deteriorating picture quality.
The circuit of FIG. 8 has been proposed to eliminate such a center/periphery brightness variation in a picture.
In the circuit of FIG. 8, input terminals 11-13 are supplied with an R (red) video signal S.sub.R, a G (green) video signal S.sub.G, and a B (blue) video signal S.sub.B, i.e., three primary color video signals. These signals are sent, via variable gain circuits 21-23 (video signal amplifying means) such as voltage-controlled amplifiers), to CRTs of the primary colors, i.e., a red-light emission CRT 31, a green-light emission CRT 32, and a blue-light emission CRT 33.
Light beams emitted from the respective CRTs 31-33 are projected onto a screen such as the screen 100 via lenses 41-43. Gain control terminals of the respective variable gain circuits 21-23 are supplied with a brightness correction control signal of a given waveform as read out from a waveform ROM 50 (storing means).
The waveform ROM 50 is supplied with a brightness adjustment signal SC.sub.Y from a terminal 53, as well as vertical and horizontal sync signals SP.sub.V and SP.sub.H from terminals 51 and 52. In general, the brightness adjustment signal SC.sub.Y is produced by manually operating, for instance, a brightness adjustment knob of the video projector. Alternatively, brightness adjustment may be performed automatically in accordance with an automatically detected light quantity at an installation location (e.g., brightness of a room) of the video projector.
In the circuit of FIG. 8, a brightness correction control signal as read out from the waveform ROM 50 is supplied to the variable gain circuits 21-23 while the screen is scanned two-dimensionally according to the vertical and horizontal sync signals SP.sub.V and SP.sub.H. As a result, gain control is effected as shown in FIG. 9 and brightness correction is performed on the screen as shown in FIG. 10.
FIGS. 9 and 10 respectively show, using as a parameter the level of the brightness adjustment signal SC.sub.Y supplied to the terminal 53, how the gain and the brightness vary in the horizontal (or vertical) direction of the screen. Curves a, b, . . . , g are obtained in this order as the level of the brightness adjustment signal SC.sub.Y decreases, i.e., as the brightness setting becomes lower. Dashed lines in FIG. 10 represent brightness values on the screen that are obtained when no brightness adjustment is performed.
In the case of curves d, e, f and g of FIG. 10 which correspond to low brightness control levels, uniform brightness is obtained over the entire display screen on which a video image is projected. That is, the unevenness in brightness indicated by the dashed lines in FIG. 10, i.e., the phenomenon that the brightness is lower in the screen peripheral portion (horizontal and vertical end portions) than in the screen central portion is compensated for by sending a brightness correction control signal (curves d, e, f and g in FIG. 9) from the waveform ROM 50 to the variable gain circuits 21-23.
As described above, the waveform ROM 50 stores a brightness correction control signal of a given waveform. In mass-producing a large number of video projectors of the same specification, the waveform ROM 50 stores the same brightness control correction signal for those video projectors with an assumption that they have approximately the same unevenness in brightness.
However, in practice, when a large number of video projectors are produced, it is not always the case that all the projectors have a maximum brightness value at the screen center. The unevenness in brightness occurs due to a variety of factors such as angular errors in mounting angles of the lenses 41-43, errors in the brightness of the CRTs 31-33, errors in the gains of the variable gain circuits 21-23, an error in the transmittance of the screen (not shown in FIG. 8), and even a brightness variation in a room where the projector is installed (e.g., a brightness difference between two horizontally distant locations). Therefore, strictly speaking, different brightness correction control signals should be prepared for respective projectors. However, it is difficult to prepare a number of correction characteristics that are to be stored in the waveform ROM 50 and select one of those characteristics for each projector.
Actually, in many cases, it is necessary to make the characteristic curves (see FIG. 9) asymmetircal so that peaks are deviated from the center rightward or leftward. That is, if the same brightness correction control signal is stored in the waveform ROM, it is difficult to equalize the brightness characteristics of a large number of mass-produced video projectors. Further, although a user can adjust the brightness for the entire screen, he cannot adjust a balance of brightness among several locations on the screen.